This invention relates to the field of presser bars for sewing machines and, in particular, to a simplified presser bar guide structure that substantially eliminates the necessity of presser foot pressure adjustment by the sewing machine operator. The invention is especially related to a presser bar bushing that need not be rigidly attached to the head of the machine and is, therefore, easy to install. The presser bar bushing of this invention is also simple to manufacture.
Various arrangements have been proposed heretofore to use tension springs to furnish the force necessary for a sewing machine presser bar. One such arrangement is shown in the copending application of James A. Transue and William Weisz, Ser. No. 81,404, filed Oct. 3, 1979 and assigned to the assignee of the present case. In that application, a tension spring is screwed onto threaded, hollow sleeves extending from members spaced along the presser bar. The force of the spring is directed along the axis of the presser bar to avoid pulling the presser bar to one side, which would produce an effect known as stick slip. The member nearer the presser foot end of the presser bar is held down by an adjustment structure that includes a yoke that prevents the member from rotating. The yoke is attached to adjustment means to adjust the pressure applied from the tension spring via the presser bar and the presser foot to work being sewn. The member onto which the other end of the tension spring is screwed is rigidly attached to the opposite end of the presser bar from the presser foot and is connected to a presser foot lever pivotally mounted on a rigid part of the machine and having a cam through which pressure is applied to lift the pressure bar and the presser foot away from the material being sewn to allow that material to be moved freely.
The apparatus to which the tension spring is connected in the Transue et al application is relatively complex to manufacture, assemble and operate. The basic concept of applying pressure axially by way of a tension spring screwed onto a threaded sleeve attached to the presser bar and another threaded sleeve attached to the machine to avoid stick slip operates quite satisfactorily, but its specific embodiment in the Transue et al application involves complex machining and assembly operations. In addition, it makes available to the sewing machine operator a presser bar pressure adjustment that we have since determined to be unnecessary and even undesirable.
The use of an extension spring to apply downward force to a presser bar to bias it against the work in a sewing machine has been suggested by Rodman in U.S. Pat. No. 823,442, by Feigel in U.S. Pat. No. 1,749,529, by Niekrawietz in U.S. Pat. No. 3,282,237, by Herr in U.S. Pat. No. 3,611,963, by Giesselmann et al in U.S. Pat. No. 4,044,701, and by Takikawa in Japanese disclosed patent application 53-141755, published Dec. 9, 1978. However, in each of those documents, the force of the extension spring was not applied directly along the axis of the presser bar but was applied to one side of the axis. This not only produced a mechanical moment resulting in mechanical hysteresis, but also made it impossible to achieve the simplified arrangement of screwing the spring onto threaded members concentric with the presser bar. A further spring adjustment mechanism was proposed by Chawick in U.S. Pat. No. 1,221,138 in which a spring was screwed onto the threaded end of a bolt rotatably supported in a sewing machine head. The other end of the spring was attached to a feeding bar to apply pressure to it, entirely separate from the presser foot.